The present invention relates to a fixing device for use in an electrophotographic image forming apparatus such as a laser printer, copying machine, facsimile, etc.
The image forming apparatus includes an image forming or developing unit where a toner image is formed on an image recording medium or sheet and an image fixing unit where the toner image is fixed to the medium. Several kinds of fixing device have been known such as a non-contact type thermal fixing method using an oven, a photo-fixing method using a flash lamp, a pressure-fixing method using a pair of pressure rollers, and a heat-contact method using a heat roller and a pressure roller.
A conventional fixing device of the heat-contact type includes a frame body having longitudinal end portions provided with bearing portions for supporting axially end portions of the heat roller and the pressure roller. The main frame is also provided with a front guide member for guiding the image recording medium fed from the image forming unit into a fixing region defined between the heat roller and the pressure roller, and a rear guide member for guiding the image recording medium from the fixing region to a discharge guide portion. After these front and rear guide members are attached to the main frame, an upper cover member which is adapted to cover an upper portion of the heat roller is attached to the main frame. Since such conventional fixing device requires great numbers of components and pieces and since neighboring components must undergo position adjustment with each other, intricate assembling work is required and increase in production cost results.
Further, the fixing device is generally positioned downstream of the image forming unit and near the sheet discharge side. If the distance between the image forming unit and the fixing device is reduced in an attempt to provide a compact image recording apparatus, temperature of a toner cartridge etc. in the image forming unit may be elevated due to the heat released from the heat roller of the fixing device, and therefore, the internal toners may be softened to degrade the imaging quality.
In order to overcome this problem, a vent fan is provided to discharge the heated air, or heat insulative partition wall is installed at a boundary between the image forming unit and the fixing device. However, such additional arrangement requires additional assembling work and increase production costs.
In another aspect, in the conventional fixing device of the heat-contact type, the sheet may be curled and wound around an outer peripheral surface of the heat roller due to the heat applied from the heat roller to the sheet. To avoid this, a sheet peel pawl or a peel guide member is attached to the main frame at a position downstream of the heat roller. The pawl or the guide member is positioned extremely close to the outer surface of the heat roller so as to separate the curled sheet from the heat roller.
However, since the imaging surface of the sheet is in slide contact with the sheet peel pawl or the peel guide member after the leading edge of the sheet is separated from the heat roller, the toners at the imaging surface may be transferred and affixed to the pawl or the guide member. If the toner deposition onto the pawl or the guide member is repeatedly carried out, the deposited toner may damage to the imaging surface due to unwanted friction or toner transfer from the deposited toner to the imaging surface. In order to avoid this problem, the peel pawl or the peel guide member can be formed of a material capable of avoiding toner deposition, or the peel pawl or the guide member is coated with the material, which in turn, causes increase in production cost.
Japanese Patent Application Kokai No. Hei 2-100059 discloses a fixing device of the heat-contact type having a pressure roller and a heat roller positioned therebelow. An outer layer of a pressure roller is formed of a resilient material. The heat roller is of a hollow shape having an aluminum sleeve and a resilient layer provided at an outer peripheral surface of the aluminum sleeve. A rod like heat source such as a halogen lamp is disposed in the hollow space of the heat roller.
In order to rotate the pressure roller and the heat roller about their axes, one of the heat roller and the pressure roller is rotationally driven. On the other hand, the halogen lamp is stationarily supported in concentrical or coaxial relation within the sleeve regardless of the rotation of the sleeve so as to uniformly heat the peripheral surface of the sleeve. The halogen lamp is fixed to a position by a fixing component. The both end portions of the halogen lamp have electrodes to which electric current feed electrodes are connected so as to supply electric current to the halogen lamp. A case or a cover is disposed over the heat roller so as to protect an operator against heat when the operator's hand is accessed nearby the heat roller for removing a jammed sheet.
However, the halogen lamp has a limited service life due to the cut-off of a wire after a long term use. Therefore, the halogen lamp must be changed with a new halogen lamp. For the exchanging work, the case or cover must be removed, and the fixing component of the halogen lamp must be removed, and the electric current feed electrodes must be disconnected from the electrodes. Thus, exchanging works requires great labor, and various tools must be used for the replacement.